**Introductory Chapter: Electronics Cooling—An Overview**

S M Sohel Murshed

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/63321

## **1. Introduction**

Recent development in semiconductor and other other mini- and micro-scale electronic technologies and continued miniaturization have led to very high increase in power density for high-performance chips. Although impressive progress has been made during the past decades, there remain serious technical challenges in thermal management and control of electronics devices or microprocessors. The two main challenges are: adequate removal of ever increasing heat flux and highly non-uniform power dissipation. According to a report of the International Electronics Manufacturing Initiative (iNEMI) Technology Roadmap [1], the maximum projected power dissipation from high-performance microprocessor chips will reach about 360 W by 2020. In fact, the micro- and power-electronics industries are facing the challenge of removing very high heat flux of around 300 W/cm2 while maintaining the temperature below 85°C [2]. Furthermore, due to increasing integration of devices, the power dissipation on the chip or device is getting highly non-uniform as a peak chip heat flux can be several times that of the surrounding area.

Conventional cooling approaches are increasingly falling apart to deal with the high cooling demand and thermal management challenges of emerging electronic devices. Thus, highperformance chips or devices need innovative techniques, mechanisms, and coolants with high heat transfer capability to enhance the heat removal rate in order to maintain their normal operating temperature. Unless they are cooled properly, their normal performance and longevity can deteriorate faster than expected. In addition, the failure rate of electronic equipment increases with increasing operating temperature. Reviews and analyses on research and advancement of conventional and emerging cooling technologies reveal that microchannel-based forced convection and phase-change cooling (liquid) are among the most promising techniques that are capable of achieving very high heat removal rates [2–6].

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

On the other hand, most of the cooling techniques cannot achieve the required performance due to the limitations in heat transfer capabilities of traditional coolants such as air, oil, water, and water/ethylene glycol/methanol mixtures, which inherently possess poor heat transfer characteristics, particularly thermal conductivity and convective heat transfer coefficient (HTC). For instance, in order to accommodate a heat flux of 100 W/cm2 at a temperature difference of 50 K, it requires an effective HTC (including a possible area enlarging factor) of 20,000 W/m2 K, which is usually not possible through free and forced convections of these coolants [7]. Thus, there is always a desperate need to find cooling fluids with superior heat transfer performance. Consequently, there are several recently emerged fluids, which can potentially be used as advanced coolants. One such fluid is nanofluid—a new class of heat transfer fluids, which are suspensions of nanometer-sized particles in conventional heat transfer fluids such as water (W), ethylene glycol (EG), oils, and W/EG. Nanofluids were found to possess considerably higher thermal properties, particularly thermal conductivity and convective as well as boiling heat transfer compared to their base conventional fluids [8–12]. With highly desirable enhanced thermal properties, this new class of fluids can offer immense benefits and potentials in wide range of applications including cooling of electronics and other high-tech industries [12–14]. Recently, another novel class of fluids—termed "ionanofluids" was proposed by our group [15–16]. Ionanofluids, which are suspensions of nanoparticles in only ionic liquids, were also found to have superior thermal properties compared to their base ionic liquids [15–17]. In addition to their unique features like green fluids and designable for specific tasks, ionanofluids show great potential as advanced heat transfer fluids in cooling electronics.

In this chapter, an overview of various cooling methods and traditional coolants for electronic devices is presented first. Then, heat transfer properties and performances of new coolants are summarised, followed by their potential in electronics cooling.

### **2. Cooling methods**

Despite impressive progress made on electronic cooling systems in recent years, the required high heat flux removal from the high-tech electronic devices remains inadequate and very challenging. There are a number of cooling methods widely used in electronic industries. Based on heat transfer effectiveness, the existing cooling modes can be classified into four general categories which are [18]:


Based on the approximate range of heat flux removal rate of these methods, it is known that liquid evaporation is the best technique followed by the forced convections of liquids and then air [18]. However, forced air convection, which is widely used in cooling electronics such as CPU of computing devices, has very low heat removal rate (though higher than radiation and natural convection). As well known, besides heat removal mode, cooling fluids also play a major role in overall cooling performance.

High-performance electronic devices and chips need innovative techniques and systems design to enhance the heat removal rate in order to minimize their operating temperature and maximize longevity. Traditional cooling approaches, consisting typically of air-cooled heat sinks, are increasingly falling short in meeting the cooling demands of modern electronic devices with high-powered densities. Thus, in recent years, various techniques for cooling such electronics have been studied extensively and employed in various thermal management systems. These include thermosyphons [19], heat pipes [20], electro-osmotic pumping [21], microchannels [4, 5], impinging jets [22], thermoelectric coolers [23], and absorption refriger‐ ation systems [24]. These cooling techniques can be categorized into passive and active systems. Passive cooling systems utilize capillary or gravitational buoyancy forces to circulate the working fluid, while active cooling systems are driven by a pump or compressor for higher cooling capacity and improved performance. As a passive cooling and given high latent heat of fusion, high specific heat, and controllable temperature stability of phase change materials (PCMs), PCMs-based heat sinks are relatively new techniques that can be used for transient electronic cooling applications [25].

Microscale cooling systems can sufficiently cool those high heat-generating electronic devices or appliances. For example, the heat transfer performances of microchannel based heat-sinks and micro-heat pipes are much higher compared to traditional heat exchangers. Because of the very compact, lightweight, suitable for small electronic devices, and superior cooling performance, microchannel-based cooling systems have received great attention from researchers and industries. The forced convective liquid cooling through microchannel heat sink is one of the promising and high-performance cooling technologies for small-sized high heat-generating electronic devices. Besides significantly minimizing the package size, this emerging cooling technology is also amenable to on-chip integration [4, 5].

Heat pipes-based electronics cooling is very popular and is recently receiving great attention from the researchers as well as industries and are already used in various electronic devices. Thus, a couple of chapters have particularly been devoted on this topic and it is not discussed here further.

On the other hand, direct liquid immersion cooling offers a very high HTC, which reduces the temperature rise of the chip surface. **Figure 1** compares the relative magnitudes (approximate) of HTCs of various commonly used coolants and cooling modes. The relative magnitude of HTC is directly affected by both the coolant and the mode of heat transfer (**Figure 1**). While water (deionized) is the most effective coolant, the boiling and condensation offer the highest HTCs.

Whatever methods are used to cool the devices or chips, transferring the heat to a fluid with or without phase transitions, it is necessary to dissipate the heat to the environment. This is mostly done with the forced convection of air, which is not sufficient particularly for high heat removal situations. Thus, it is also of tremendous importance to efficiently take away the heat from the coolants.

**Figure 1.** Range of overall heat transfer coefficients for different fluids and cooling modes.

## **3. Cooling fluids**

#### **3.1. Conventional coolants**

There are a number of aqueous and non-aqueous conventional coolants which are used in various electronics cooling systems. As water possesses higher thermal conductivity and specific heat and lower viscosity compared to other coolants, it is the most widely used coolant for electronics. But water is not used in closed loop systems due to its high freezing point and the expansion upon freezing.

Nonetheless, it is important to select the best coolant for any specific device or cooling system. There are some general requirements for coolants and they may vary depending on the type of cooling systems and electronic devices. As well discussed in the literature [26], the liquid coolants for electronics cooling must be non-flammable, non-toxic, and inexpensive with excellent thermophysical properties and features, which include high thermal conductivity, specific heat and HTC, and low viscosity. Besides good chemical and thermal stability, coolants must also be compatible (e.g., non-corrosive) with the materials of the components of the cooling systems and devices. However, selection of a coolant for direct immersion cooling cannot be made only based on the heat transfer features. Chemical compatibility of the coolant with the chips and other packaging materials must be considered as well. The commonly used coolants for electronics cooling are mainly classified into two groups: dielectric and nondielectric coolants.

There are several types of dielectric coolants, which are aromatics, aliphatics, silicones, and fluorocarbons-based fluids. Aromatics coolants such as diethylbenzene (DEB), toluene, and benzenes are the most commonly used coolants. Aliphatic hydrocarbons of paraffinic and isoparaffinic types (including mineral oils) and aliphatic polyalphaolefins (PAO) are used in a variety of direct cooling of electronics. Silicones-based coolant is another popular type of coolant widely known as silicone oils, e.g., Syltherm XLT. The fluorocarbons series of coolants such as FC-40, FC-72, FC-77 and FC-87 are widely accepted in the electronics industries.

Non-dielectric liquids are also used for electronics cooling because of their better thermal properties compared to their dielectric counterparts. They are normally aqueous solutions and thus exhibit high heat capacity and thermal conductivity. Water, EG, and mixture of these two (W/EG) are very popular and widely used as electronics coolants. Other popular non-dielectric coolants include propylene glycol (PG), water/methanol, W/ethanol, NaCl solution, potassium formate (KFO) solution, and liquid metals (e.g., Ga-In-Sn). Mohapatra and Loikits [26] evaluated that among the various coolants, KFO solution possesses highest overall efficiency. Comparisons of various properties and characteristics of all types of available coolants can help selecting the right coolants.

#### **3.2. Potential new coolants**

As mentioned before, the cooling demands of modern electronics devices or systems cannot be met by those conventional coolants due to their inherently poor thermal properties which greatly limit the cooling performance. Here, the newly emerged heat transfer fluids like nanofluids and ionanofluids, which have highly desirable superior thermal properties and are suitable for even microsystems, can be the cooling solutions. These new fluids can also offer immense benefits and potential applications in a wide range of industrial, electronics, and energy fields [12–14, 17]. Results of key heat transfer features including thermal conductivity, convective and boiling of these new coolants are briefly summarized in the following subsec‐ tion.

#### *3.2.1. Summary of thermal properties and performance*

Extensive research has been performed on the thermal conductivity of nanofluids and studies showed that nanofluids possess considerably higher thermal conductivity compared to their base fluids [8, 12, 27–28]. However, results from different research groups are not very consistent and sometimes also controversial particularly regarding the heat transfer mechanisms [29]. Nanofluids also exhibit superior other thermophysical properties than those of base fluids [8, 27, 30–32]. With significantly high thermal properties, nanofluids can meet the cooling demand of high-tech electronics devices.

Evaluating the convective heat transfer performance of nanofluids is very important in order for their application as coolants in electronics. There have been large number of studies on convective heat transfer of nanofluids and nanofluids are found to exhibit enhanced HTC compared to their base fluids at any flow conditions. The enhanced HTC (*h* or *Nu*) further increases considerably with increasing concentration of nanoparticles as well as Reynolds number (*Re*) or flow rate [9, 33–34]. The enhancement of HTC is even more significant at turbulent regime. Based on the findings of convective heat transfer, it is considered that nanofluids can perform better cooling compared to conventional fluids in electronics cooling systems.

Another very important and efficient mode of cooling is boiling or phase change of fluids in various heat exchange systems. There is an increasing research focus on this key-cooling feature of nanofluids. Studies on boiling heat transfer of nanofluids revealed an undisputed substantial increase (up to few times of base fluids) in the boiling critical heat flux of nanofluids [9, 35–36]. Research also demonstrated that the boiling performance of nanofluids can be enhanced further with nanoparticle concentration and various other factors such as deposition of nanoparticles on heater wall, roughness of wall surface, and addition of surfactant [35–38]. Given the superior convective and boiling heat transfer performances, these new fluids can considerably increase the HTC and can act as better coolants than water or other conventional coolants.

Like nanofluids, ionanofluids also exhibit superior thermal properties, particularly thermal conductivity and heat capacity compared to their base ionic liquids [15–17]. Besides good thermal stability, thermophysical properties of ionanofluids can be adjusted by changing the ionic composition and structure of base ionic liquids. Early research revealed that these new nanofluids showed great potential to be used as advanced coolants for electronics cooling [16– 17].

#### *3.2.2. Potential of new fluids in electronics cooling*

In recent years, extensive research works have been performed on the application of micro‐ channel cooling systems (e.g., heat sinks) for electronics cooling [4, 5, 39]. Since the convective HTC is inversely proportional to the hydraulic diameter of the channel, very high heat transfer performance can be achieved by using microchannel at any flow regime. The forced convective heat transfer of cooling fluids through microchannel heat sinks is among the more promising technologies, which can offer very high heat removal rates [4, 5, 21, 39]. Nevertheless, the main limitation of cooling performance actually raised from the low heat transfer capability of the coolants used. In this regards, nanofluids with superior heat transfer performance can potentially boost the heat removal performance of microchannel cooling systems even further and be able to remove high heat flux of high-tech electronics devices.

Nanofluids have directly been employed in cooling systems of electronic or computing devices to evaluate the performance of these new fluids [40–42]. Results were very promising as the application of nanofluids in those cooling systems resulted in better cooling performance compared to traditional base fluids [39–43]. Thus, applications of nanofluids in conventional and emerging techniques such as microchannels and heat pipes can be the next-generation electronics cooling systems. A detailed discussion and analysis on the potential benefits and applications of nanofluids in cooling electronics can also be found in an ongoing study by the author [44].

## **4. Conclusions**

Advances in electronics and semiconductor technologies have led to a dramatic increase in heat flux density for high-performance chips and components, whereas conventional cooling techniques and coolants are increasingly falling short in meeting the ever-increasing cooling need of such high heat-generating electronic devices or microprocessors. Despite good progress been made during the past decades, there remain some serious technical challenges in thermal management and cooling of these electronics. High-performance chips and devices need innovative mechanisms, techniques, and coolants with high heat transfer capability to enhance the cooling rate for their normal performance and longevity. With superior thermal properties and cooling features, nanofluids offer great promises to be used as coolants for hightech electronic devices and industries. The emerging techniques like microchannels with these new fluids can be the next-generation cooling technologies.

## **Author details**

S M Sohel Murshed

Address all correspondence to: smmurshed@ciencias.ulisboa.pt

Faculty of Sciences, University of Lisbon, Lisbon, Portugal

## **References**


## **Boiling of Immiscible Mixtures for Cooling of Electronics**

Haruhiko Ohta, Yasuhisa Shinmoto, Daisuke Yamamoto and Keisuke Iwata

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/62341

#### **Abstract**

To satisfy the requirements for the cooling of small and large semiconductors operated at high heat flux density, an innovative cooling method using boiling heat transfer to immiscible liquid mixtures is proposed. Immiscible liquid mixtures discussed here are composed of more-volatile liquid with higher density and less-volatile liquid with lower density, and appropriate volumetric ratios become a key to realize high-performance cooling. The chapter reviews the experimental results obtained by the present authors, where critical heat flux accompanied by the catastrophic surface temperature excursion is increased up to 300 W/cm2 for FC72/water by using a flat heating surface of 40 mm in diameter facing upwards under the pressure 0.1 MPa.

To apply the superior heat transfer characteristics in boiling of immiscible mixtures to flow boiling system, preliminary experiments using a horizontal heated tube are performed and the classification of flow pattern with liquid-liquid interface and corresponding heat transfer performance are discussed.

**Keywords:** cooling of electronics, immiscible mixture, insoluble mixture, pool boiling, flow boiling

#### **1. Introduction**

#### **1.1. Possibility of non-azeotropic mixtures**

For the systems of air conditioning and refrigeration, non-azeotropic miscible mixtures are often used as the working fluids alternative to the discontinued ones. However, these fluids have a well-known unavoidable disadvantage of heat transfer deterioration resulting from the increased interfacial temperature due to the existence of mass diffusion resistance. At the same

time, for the non-azeotropic miscible mixtures, there is an unknown effect of Marangoni force exerted mainly by the concentration difference along the liquid-vapor interface as a result of the preferential evaporation of more-volatile component. In aqueous solutions of alcohol with a large carbon number, the surface tension is increased with increasing temperature depending on the range of concentration and the level of temperature. In such a condition, Marangoni force due to the concentration gradient is enhanced by also the temperature gradient along the interface, especially near the three-phase interline extended at the base of bubbles. The enhancement of critical heat flux (CHF) was shown by Vochten-Petre [1] and Van Stralen [2] based on the experiments using a wire heater. Abe [3] verified the drastic increase of maximum heat transportation for heat pipes using "self-rewetting mixtures". Sakai et al. [4] confirmed the small enhancement of heat transfer in the ranges of very low alcohol concentration in water, while no appreciable increase in CHF for a flat heating surface facing upwards. As a conse‐ quence, non-azeotropic mixtures have no advantage from the view point of the improvement of heat transfer.

#### **1.2. Expected performance of immiscible mixtures**

Innovative cooling systems which meet the requirement for the increased heat generation density from electronic devices are urgently required. To enhance the values of CHF for the cooling of a large area at high heat flux larger than 200 W/cm2 as shown in **Figure 1**, the present authors confirmed the validity of the devised structure which reduces the effective heating length by the liquid supply directly to the downstream of the heating duct from the transverse direction. An example of the structure is illustrated in **Figure 2** [5,6]. However, such structure is rather complicated. On the other hand, to ensure the high reliability for a long-term operation, microstructures on the "enhanced surface" cannot be accepted depending on their application. The present authors noticed the superior heat transfer characteristics in nucleate boiling of immiscible mixtures even on a smooth surface (Kobayashi et al. [7], Ohnishi et al. [8], Kita et al. [9]), which are summarized as follows.


Boiling of Immiscible Mixtures for Cooling of Electronics http://dx.doi.org/10.5772/62341 13

**Figure 1.** Difference in the size and heat flux level of semiconductors as a target of cooling.

**Figure 2.** Devised structure of cold plate for the cooling by flow boiling in a narrow channel [5,6].

#### **1.3. Existing researches on boiling of immiscible mixtures**

A large number of reports on nucleate boiling of oil mixtures exist. Filipczak et al. [10] used emulsions of oil and water, where the distribution of two liquids and vapor was investigated at different levels of heat flux. The heat transfer coefficients for high oil concentration were quite smaller than those for pure water, because the free convection of oil dominates the heat transfer to water-oil mixture. At the initial stage of nucleate boiling, foaming was observed before the formation of emulsion. Roesle and Kulacki [11] studied nucleate boiling of FC72/ water and pentane/water on a horizontal wire. The discontinuous phase of more-volatile components FC72 and pentane were dispersed in a continuous phase of water, where the concentrations of more-volatile component were varied as 0.2–1.0% and 0.5–2.0%, respectively. Nucleate boiling of dispersed component or of dispersed and continuous components was observed depending on the level of heat flux. The heat transfer was enhanced by nucleate boiling of dispersed liquid if its volume fraction was larger than 1%. Bulanov and Gasanov [12] studied the heat transfer to four emulsions, n-pentane/glycerin, diethyl ether/water, R113/ water and water/oil, where more-volatile liquids were dispersed in the continuous phase of less-volatile liquids. The reduction of surface superheat at the boiling initiation was observed compared to that for pure less-volatile liquids.

On the other hand, the number of investigations on immiscible mixtures which form stratified layers of component liquids before the heating is quite limited. There are old studies by Bonilla and Eisenbuerg [13], Bragg and Westwater [14], Sump and Westwater [15]. Bragg and Westwater [14] classified heat transfer modes for individual layers of liquids. The interpreta‐ tion of data, however, was not described in detail. Gorenflo et al. [16] studied boiling of water/ 1-butanol on a horizontal tube, where the liquid mixture becomes soluble or partially soluble depending on its concentration, and levels of temperature and pressure. From the experiments performed under various combinations of concentration and pressure, they reported that the nucleate boiling heat transfer is not largely depending on the solubility.

## **2. Immiscible mixtures**

#### **2.1. Phase equilibrium**

Immiscible mixtures employed here consist of insoluble component liquids and their phe‐ nomena during nucleate boiling have a unique feature characterized by self-sustaining subcooling of liquids. An example of phase equilibrium diagrams for FC72/water at the total pressure of 0.1 MPa is shown in **Figure 3**. The concentration where the two curves merge, which is corresponding to the azeotropic point frequently observed in miscible mixtures, is the concentration of vapor phase independent of the liquid composition for immiscible mixtures. The concentrations *Y*1 and 1−*Y*1 (=*Y*2) on the dew point curves for lower and higher concentrations of more-volatile component in liquid phase are calculated by the following equations, respectively (e.g., Prigogine and Defay [17]).

$$\ln Y\_1 = -\frac{h\_{\beta\varepsilon,1}}{R\_1} \left( \frac{1}{T} - \frac{1}{T\_{\text{sat},1}} \right) \tag{1}$$

$$\ln\left(1 - Y\_1\right) = -\frac{h\_{\beta\varepsilon,2}}{R\_2}\left(\frac{1}{T} - \frac{1}{T\_{\text{sat},2}}\right) \tag{2}$$

where *Y*1: mole fraction of more-volatile component in vapor on dew point curve [−], *T*: dew point temperature [K], *Tsat*: saturation temperature [K] for a given total pressure, *hfg*: latent heat of vaporization [kJ/kg] and *R*: gas constant [kJ/kg·K]. The equations are easily derived from

the Clausius-Clapeyron equation and the ideal gas relation applied approximately to vapor phase.

**Figure 3.** Phase equilibrium diagram of FC72/water mixture.

#### **2.2. Conditions of component liquids**

The conditions of liquid phase are represented in **Figure 4**, where two saturated vapor pressure curves with a red line for a more-volatile component and a blue line for a less-volatile one are drawn. Since immiscible mixtures have the total pressure *Ptotal* as the sum of saturated vapor pressures corresponding to the equilibrium temperature *Te*, the equilibrium state of the mixture is represented by a black point in the figure and the relation *Psat*,1(*Te*) + *Psat*,2(*Te*) = *Ptotal* holds true. Immiscible liquids are separated because of the difference in their densities, and one component liquid contacts or tends to contact the surface located at the bottom. The equilibrium temperature of immiscible mixtures is realized by the evaporation of both components. The degree of subcooling becomes the difference between the saturation tem‐ perature of each component corresponding to the total pressure and the equilibrium temper‐ ature of the mixture. If either of two components is not evaporated enough or not satisfy the saturation state corresponding to the equilibrium temperature but the vapor of one component is superheated, the liquid state of the other component is deviated from the equilibrium state represented in the figure. The equilibrium temperature of mixtures tested here and the degree of subcooling for each component liquid are shown in **Tables 1** and **2**, respectively. The subcooling of less-volatile liquid excessively compressed by the high vapor pressure of morevolatile component becomes very high, while the subcooling of more-volatile liquid is very low.

**Figure 4.** Vapor pressure curves of components for immiscible mixtures.


**Table 1.** Saturation temperature of each component and equilibrium temperature of immiscible mixtures at 0.1 MPa


**Table 2** Degree of subcooling for component liquids at 0.1 MPa

### **3. Pool Boiling**

#### **3.1. Experimental apparatus for pool boiling**

The outline of the apparatus is shown in **Figure 5**. A flat heating surface of 40 mm in diameter made of copper is located horizontally facing upwards. The upper surface of cylindrical copper heating block is operated as the heating surface surrounded by a thin fin cut out in one unit body to prevent the preferential nucleation at the periphery. Nineteen cartridge heaters are inserted in the heating block and the maximum amount of heat generation is 5700 W. Eight thermocouples are inserted in the center and side of copper heating block at four different depths of 1, 7, 13 and 19 mm below the heating surface to estimate the heat flux and heating surface temperature. Fluid temperatures are measured by three thermocouples at 2, 80 and 160 mm above the heating surface in the boiling vessel, where the liquid level is located between the second and third thermocouples.

**Figure 5.** Outline of pool boiling experimental apparatus.

The experiments are performed at 0.1 MPa changing volume ratio of the components. Immiscible mixtures of FC72/water and Novec7200/water are used as test fluids, where FC72 and Novec7200 are more-volatile components with higher density and water is less-volatile one with lower density. The conditions for the volume ratio of component liquids are represented by **Figure 6** [9], where *H*1 is the height of the more-volatile liquid from the heating surface and *H*<sup>2</sup> is for the less-volatile liquid. The total height is kept at 100 mm, i.e., *H*1 + *H*2 = 100mm, and tested compositions are listed in **Table 3**.

**Figure 6.** Condition representing volume ratio of immiscible liquids [9].


**Table 3.** Tested composition of liquids

#### **3.2. Experimental results for pool boiling**

Experimental results are shown in **Figures 7** and **8** for FC72/water and Novec7200/water, respectively. **Figure 7** represents the relation between heat flux q and heating surface temper‐ ature *Tw* for FC72/water, where representative heat transfer characteristics of immiscible liquids are known. Independent of volume ratios, the heating surface temperatures for mixtures are located between those for pure liquids. The curve for [50 mm/50 mm] almost coincides with that for the saturated boiling of FC72. For [10 mm/90 mm], a temperature jump similar to burnout phenomena occurs, which is referred to as the "intermediate heat flux burnout" by the present authors. For [5 mm/95 mm] and [0 mm/100 mm], the surface temper‐ ature increases with heat flux, where the heat transfer mode changes from natural convection to nucleate boiling of water under high subcooled conditions. For [5 mm/95 mm], the reduction of surface temperature from that of saturated nucleate boiling of water is clearly observed at high heat flux due to the heat transfer enhancement resulting from the generation of bubbles composed mainly by FC72 vapor. For [0 mm/100 mm], the value of CHF increases from 1.35 MW/m2 of saturated water to 3.04 MW/m2 of FC72/water mixtures at 0.1 MPa. The marked increase of CHF resulted from the high subcooling of water as much as 48.4 K due to the excessive compression by FC72 vapor. Similar results are obtained also for Novec7200/water despite of the quantitative difference in the effect of liquid height between the two mixtures tested here. In **Figure 9**, the values of CHF are compared with those of subcooled boiling of

**Figure 7.** Heat flux versus heating surface temperature for FC72/water.

**Figure 8.** Heat flux versus heating surface temperature for Novec7200/water.

pure water estimated by Ivey-Morris correlation [18]. For [0 mm/100 mm], the experimental CHF values are close to the predicted ones, while the discrepancy is increased as the liquid height of more-volatile component increases. This is because boiling of the more-volatile component promotes the coalescence of bubbles and the dryout occurs at lower heat fluxes.

**Figure 9.** The comparison of CHF data with predicted values for subcooled boiling of less-volatile liquid.

#### **3.3. Consideration on the mechanisms of intermediate heat flux burnout**

The phenomena of limited jump of heating surface temperature referred here to as the intermediate heat flux burnout occurs if the thickness of the more-volatile liquid with higher density attached to the horizontal heating surface is small as observed for FC72/water [10 mm/ 90 mm] at *q* = 2.0 × 105 W/m2 in **Figure 7** and Novec7200/water [5 mm/95 mm] at *q* = 2.3 × 105 W/m2 in **Figure 8**.

After the jump of the surface temperature, the heat transfer mode is changed from the nucleate boiling of more-volatile liquid to the natural convection or nucleate boiling of less-volatile liquid at higher heat fluxes. It is very important that the generation of bubbles or vapor slugs of more-volatile component continues also in this region enhancing the heat transfer to the less-volatile liquid. The intermediate heat flux burnout is observed when Taylor instability occurs by the growth of a coalesced bubble with more-volatile component after its lateral coalescence below the bulk of less-volatile liquid with lower density.

The value of minimum bubble diameter *dmin* penetrating across the liquid-liquid interface is evaluated from the minimum volume of a bubble *Vmin,* assuming the sphere bubble shape (Greene et al. [19], Onishi et al. [20]).

$$V\_{\min} = \left[\frac{3.9\sigma}{\text{g}\left(\rho\_{L2} - \rho\_{V1}\right)}\right]^{\frac{3}{2}}\tag{3}$$

where *σ*: interfacial tension [m/s], *g*: gravitational acceleration [m/s2 ], *ρ<sup>L</sup>*<sup>2</sup> : density of upper liquid, i.e., less-volatile liquid with lower density [kg/m3 ], *ρV*1: density of lower vapor, i.e., vapor mainly of more-volatile component [kg/m3 ]. The correlation implies that the penetration criteria is determined by two conflicting forces of the buoyancy acting upwards on a bubble and of the interfacial tension to suppress the bubble penetration. **Table 4** listed the most dangerous wavelength of Taylor instability *λd*, minimum bubble penetration diameter *dmin* and bubble departure diameter *db* under pool boiling conditions evaluated at 0.1 MPa.


**Table 4.** Wavelength for Taylor instability and minimum bubble diameter for more-volatile component to penetrate into upper less-volatile liquid at 0.1 MPa

Bubbles of more-volatile component grow by the evaporation or lateral coalescence in the vicinity of liquid-liquid interface and become sizes beyond *dmin*. The enlarged bubbles penetrate into the less-volatile liquid layer accompanying the entrainment of more-volatile liquid as shown in **Figure 10**. Under this condition, bubbles of more-volatile component do not grow to the size of the wavelength *λd*, and no mixing of liquids occurs in the vicinity of heating surface. The penetration of generated bubbles thorough the liquid-liquid interface delays their coalescence. However, at a certain value of heat flux, the rate of bubble generation exceeds the elimination of bubbles by the penetration, and the lateral bubble coalescence occurs under the liquid-liquid interface. The diameter of flattened bubble radius exceeds the wave length of Taylor instability *λd*, and the less-volatile liquid descends and starts to contact the heating surface. The large subcooling of less-volatile liquid is enough to suppress the excessive jump of heating surface temperature.

**Figure 10.** Expected behaviors of bubble in the vicinity of liquid-liquid interface.

There is another type of intermediate heat flux burnout confirmed by the present authors, where the heating surface temperature gradually deviates from that for nucleate boiling of pure more-volatile liquid [9]. In such a case, bubbles can coalesce easily below the liquid-liquid interface because of thicker thickness of its layer and exceed the value of most dangerous wave length at lower heat flux. As a consequence, the less-volatile liquid starts to contact partially the heating surface, and the contribution of the heat transfer to the less-volatile liquid gradually increased as the increase of heat flux keeping the steady-state conditions at each heat flux level. Similar phenomenon occurs in the cases of smaller wavelength of Taylor instability or of partially soluble combination of liquids depending on the selection of component liquids. It is clear that the physical and/or chemical mixing of component liquids is a key factor to determine the heat transfer characteristics at low heat flux.

## **4. Flow boiling**

#### **4.1. Experimental apparatus for flow boiling**

**Figures 11** and **12** show the outline of test section and test loop [21]. Test loop is composed of test section, condenser, liquid-vapor separation tank, circulating pump, pre-heater. Immiscible liquids are stratified in liquid-vapor separation tank, and both flow rates are controlled by valves. The test section is composed of a heated section of stainless tube spirally coiled by sheath heaters on the outer surface and a transparent unheated section of Pyrex glass for the observation of liquid-liquid and liquid-vapor interfaces. The inner diameter of both tubes is 7 mm and heated length is 310 mm. Six thermocouples are inserted in the top and bottom tube walls at the upstream, midstream and downstream locations. The experiments are conducted for the combination of FC72 and water, i.e. more-volatile component with higher density and less-volatile one with lower density whose saturation temperatures as pure components are 55.7 and 100°C, respectively, at 0.1 MPa.

**Figure 11.** Outline of test section [21].

#### **4.2. Experimental results for flow boiling**

Various flow patterns such as stratified flow, FC72 slug flow, emulsion-like flow, "wavy stratified + FC72 droplet flow", "FC72 churn + FC72 droplet flow" and "FC72 slug + FC72 droplet flow" are observed depending on the combinations of flow rates for both components under unheated conditions, and they are summarized as a liquid-liquid flow pattern map in **Figures 13** and **14** [21].It is known by the preliminary experiments that the heat transfer characteristics at low heat fluxes are strongly influenced by the liquid-liquid behaviors under unheated conditions.

Boiling of Immiscible Mixtures for Cooling of Electronics http://dx.doi.org/10.5772/62341 23

**Figure 13.** Typical flow patterns of FC72/water [21].

**Figure 14.** Flow pattern map for FC72/water. [21]

**Figure 15.** Outlet fluid temperature versus heat flux.


**Table 5.** Liquid-vapor behaviors for flow boiling of FC72/Water

To evaluate the local heat transfer coefficients, the distribution of mixture temperature along the tube axis is needed. Outlet mixture temperature can be reproduced by the heat balance equations which introduce a parameter *ξ* representing the ratio of heat supplied to morevolatile component FC72 to the total. **Figure 15** shows the outlet mixture temperature versus heat flux. The solid lines and broken lines are calculated and experimental outlet temperatures, respectively. The parameter *ξ* depends only on the flow rate ratio and not on the heat flux. This could be possible if the liquid-liquid flow pattern is not strongly dependent on the bubble generation at low heat flux as shown in **Table 5**. The error of the prediction is less than ±1.0K.

**Figures 16** and **17** show heat transfer coefficient and wall temperature versus heat flux at midstream averaged by the top and the bottom values. Heat transfer coefficient is higher and wall temperature is lower than pure water for immiscible liquid because the convection of water is enhanced by the generation of FC72 bubbles.

**Figure 16.** Heat transfer coefficient versus heat flux at midstream.

**Figure 17.** Wall temperature versus heat flux at midstream.

#### **5. Conclusions**

To clarify the boiling heat transfer characteristics of immiscible mixtures, experiments of pool boiling and flow boiling in a tube were conducted, and the following results were obtained.


For the cooling systems in an enclosure, the distribution of liquid layers for both immiscible components becomes a key to determine the heat transfer characteristics due to nucleate boiling. For a vertical heating surface or a heating surface operated under the reduced gravity conditions, some methods to transfer the heat to the more-volatile liquid with larger density are needed to obtain the superior cooling characteristics.

### **6. Nomenclature**


*Ptotal* total pressure, N/m<sup>2</sup>


#### **Greek symbols**


#### **Subscripts**


## **Author details**

Haruhiko Ohta\* , Yasuhisa Shinmoto, Daisuke Yamamoto and Keisuke Iwata

\*Address all correspondence to: ohta@aero.kyushu-u.ac.jp

Department of Aeronautics and Astronautics, Kyushu University, Fukuoka, Japan

#### **References**

